White rust is a minor disease of sweetpotato, present only on certain cultivars.

There are no records about the importance of the disease on yield.

Parts of Asia, Caribbean, Europe, Pacific Islands,  South America  and  North America.  Countries recorded include Australia, Azerbaijan, Cuba, Dominica, Dominican Republic, France, French West Indies, Guyana, Haiti, India, Italy, Jamaica, Malta, Morocco, Pakistan, Peru, Puerto Rico, Saint Vincent and the Granadines, Sudan, Turkmenistan, USA, and Uzbekistan.

The most obvious symptom is the presence of chlorotic or yellowish blotches, initially roundish to angular where they are limited by veins, on the upper surface of leaves. On the lower surface, small pustules develop which later open and expose whitish masses of sporangial pustules.  After sporulation, the infected tissue dies, forming irregular-shaped brown lesions.

In some cultivars or growing conditions, infection induces the development of galls of raised, thickened tissue.  Galls may develop on leaves, petioles, stems and flowers.  When pustules erupt, the galls become covered in the white spores.

Diseased plants can also present general distortion, defoliation and flower abortion. Distortion may occur where galls form on any part of the stem. The disease may cause twining while twining species may assume an upright habit.

In some very sensitive cultivars, symptoms resembling witches’ broom, with shortening of internodes and bunchy growth habit, have been observed.

Morphology

The mycelium is intracellular with typical knob-like haustoria. The sori are amphigenous or caulicolous, white or light yellow, prominent, superficial, measure 0.5-2.0 mm, rounded, often confluent and frequently producing marked distortion of the host. The sporangiophores are hyaline, club shaped, unequally curved at the base and measure 15 x 30 µm. The sporangia are produced in chains. They are short, cylindrical, with more rounded terminal, hyaline, and smooth; the membrane with an equatorial thickening, is usually very pronounced, measuring 14-20 x 12-18 µm. The oosporic sori are separated from the sporangial caulicolous, rarely on the petioles, measure 1-2 x 5-6 cm or even more causing marked distortion of the host. The oospores are light yellowish brown, 25-55 µm in size, and are epispore papillate or with irregular, more or less curved ridges.

Biology and ecology

During the growing season, the disease is spread by air borne sporangia after landing on the  plant surface. Germination can be direct such that the sporangia produce an infecting hypha which penetrates the plant tissue, develops intercellulary and sends round haustoria inside the cells. There is indirect germination when the sporangium liberates biflagellate zoospores that swim in a film of water and invade the cells.

Oospores overwinter in crop refuse (leaves and stems) in the field and are responsible for primary infection of sweetpotato plants the following growing season.

There is no information about the environmental factors that favour white rust development in sweetpotato. The only factor mentioned was rainfall. In other Oomycetes, the free water on plant surfaces determines the way in which the sporangia will germinate. Hence, with direct germination one sporangium is one infection point but if germination is through indirect germination, each zoospore is one infection point and every sporangium contains several zoospores.

In the case of A. ipomoeae-panduratae, germination directly or indirectly occurs optimally at 12-18°C. Infection occurs through stomata during periods of rain and cool temperatures.

Distinct races of A. ipomoeae-paduratae are found on Ipomoea batatas and other Ipomoea spp. These races morphologically differ from each other.

Host range

Ipomoea batatas (sweetpotato) is the primary host. Other hosts are members of the Convolvulus family (morning glory) such as: I. pandurata, I. pentaphylla, I biloba, I. horsfalliae, I. purpurea, I. reptans (water spinach), and Calonyction aculeatum. The disease has also been observed in some members of the family Amaranthaceae such as Amaranthus albus (white pigweed).

Detection and inspection

The symptoms in the field are so evident, that no special diagnosis is needed.

Management

No control methods are mentioned in the literature, apparently because infections are not serious enough.

In extreme cases copper fungicides can be used, as for other fungi of the same group.

Holliday, P. 1980. Fungus Diseases of Tropical Crops. Dover Publications Inc. New York 607 p.

Mukerji, K.J. and Chrichet, C. 1975. Albugo ipomoeae- panduratae. CMI Descriptions of Pathogenic Fungi and Bacteria No. 459.

Contributed by: Teresa Ames

Taxonomy

Economic importance

Geographical distribution

Symptoms

Morphology

Biology and ecology

Host range

Detection and inspection

Management

References

Chlorotic patches and lesions at various stages of development (C. Clark).

Erupted pustules on the lower leaf surface (C. Clark).

Yellowish galls as early symptom of the disease (C. Lopes/EMBRAPA).

Galls turn white as disease develops (C. Lopes/EMBRAPA).